Many aircraft use various leading edge devices to improve airfoil performance at high angles of attack. For example, modern commercial transport category aircraft generally have wings that are optimized for high speed cruise conditions. In order to improve takeoff and landing performance, these aircraft typically employ moveable leading edge devices that have at least one position, typically referred to as a retracted position, that provides optimum cruise performance, and one or more additional positions, typically referred to as extended positions, for low speed operations. The extended positions improve airflow over the airfoil during low speed operating conditions, allowing the aircraft to achieve higher angles of attack without stalling. This results in lower stall speeds for the specified configuration. Because operating speeds for takeoff and landing are typically based on a percentage of stall speed, these lower stall speeds result in improved takeoff and landing performance. Examples of typical leading edge devices include nose flaps, fixed slots, Kruger flaps, slats, and variable camber Kruger flaps. Other aircraft use leading edge devices to improve airfoil performance during other phases of operations. For example, fighter type aircraft often use leading edge devices during maneuvering flight.
FIG. 1 is a partially schematic top view of a conventional aircraft wing 1 with various control surfaces. The control surfaces include trailing edge high lift devices 4 (e.g., common flaps and fowler flaps), and leading edge devices 5 (such as those discussed above). The wing 1 also has a span 2, which is the distance from the fuselage 16 to the wing tip 17 (alternately, the span 2 can be measured from the wing tip 17 to an opposite wing tip and a semi-span can be defined as the distance from the wing tip 17 to a centerline of the fuselage 16). The leading edge devices 5 have a plurality of spanwise locations, each spanwise location having a corresponding leading edge device chord length. For purposes of illustration, one spanwise location 6 with a leading edge device chord length 7 is shown in FIG. 1, using a typical convention. In other conventions, the leading edge device chord length can be measured perpendicular to the direction that the span 2 is measured.
The wing 1 typically has at least one critical portion where the local maximum lift coefficient first occurs as the aircraft angle of attack is increased. As the aircraft angle of attack is further increased, the local maximum lift coefficient is exceeded on that portion of the wing 1, and that portion of the wing 1 becomes stalled. While the location of the critical portion of the wing can vary with design, on a typical modern swept wing transport category aircraft, it is not uncommon for the critical portion to be at approximately a 75% spanwise location (e.g., a distance from the fuselage 16 along the span equal to 75% of the distance from the fuselage 16 to the wing tip 17).
The typical design process, which yields the design depicted in FIG. 1, includes determining the amount of lift that the wing 1 must provide during various phases of flight, and an aircraft angle of attack that will be required to generate this lift. Because longer leading edge device chord lengths generally provide better high angle of attack performance, a leading edge device chord length that will support the required aircraft angle of attack on the critical portion of the wing 1 is determined. Generally, this leading edge device chord length determined for the critical portion of the airfoil is then used for all portions of all leading edge devices on the airfoil (i.e., each leading edge device has the same, constant chord length). Occasionally, a smaller chord length is used (for installation reasons) near the wing tip 17 due to spanwise wing taper or other structural constraints.
One aspect of the prior art design discussed above and shown in FIG. 1 is that the leading edge device chord length is optimized for the critical portion of the airfoil. A drawback of this aspect is that it creates a potentially inefficient design that unnecessarily increases the weight of the aircraft.